Project Summary Age-related diseases are the major causes of morbidity and mortality in Western society. Calorie restriction (CR), a dietary intervention which extends lifespan while delaying or preventing age-related disease, is one plausible approach to lessen the burden of these diseases, but reduced-calorie diets are notoriously difficult to sustain. Recent studies have highlighted an important role for dietary protein in health and longevity, with low protein (LP) diets shown to promote longevity and to mimic the metabolic benefits of CR. We have found that specifically reducing dietary consumption of the three branched-chain amino acids (BCAAs) ? leucine, isoleucine, and valine ? promotes metabolic health in both young and aged mice, and promotes longevity in progeroid mice. Determining the effect of Low BCAA diets on longevity, and identifying the physiological and molecular mechanisms by which dietary BCAAs influence metabolism, will provide mechanistic insight into the regulation of healthy aging. Eventually, this will permit the development of new pharmacological approaches to prevent or delay age-related diseases. Here, we will rigorously test the hypothesis that a Low BCAA diet can promote health and longevity in mice, examining the effect of long-term Low BCAA diet on lifespan, metabolic health, and frailty, and testing the efficacy of short-term Low BCAA diet feeding as a late-life intervention. We will determine if reduced consumption of the BCAAs mediates the metabolic effects of CR and LP diets. Using a mouse model of reduced BCAA catabolism, we will identify the distinct roles played by the BCAAs as substrates for building proteins, and as a source for metabolically active BCAA catabolites. Finally, we will examine the cell autonomous effects of the BCAAs and their catabolites on the function and metabolism of primary hepatocytes and pancreatic islets ex vivo using novel metabolic sensors, and test candidate molecular mediators of these effects. The proposed work will examine the role of the branched-chain amino acids on health and longevity for the first time, and address long-standing questions regarding how dietary quality ? the specific composition of the diet ? impacts healthy aging. Importantly, we will gain new insight into the physiological and molecular mechanisms that drive the potent effects of CR and LP diets. In the long term, this work will enable our laboratory and others to develop a mechanistic understanding of how dietary branched-chain amino acids and other macronutrients regulate health and disease vulnerability, and to identify new targets for the pharmacological treatment of age-related diseases and the promotion of healthy aging.